I bought a condo, and I didn't know the official name of this document.
I, and many others, just know that you need proof that the money has to come from abroad.
So maybe when you ask people, just ask them if you have cash in Thailand if that is good enough to buy. How about that document that the money has to come from outside of Thailand.
As far as I know you actually need that document at the land department. So maybe the best place to ask is the land department.